Motoneurons have been studied intensively for years with physiological methods, but relatively little is known about their ultrastructure. Our principal objective is to study the fine structure of soleus and plantaris motoneurons in order to type these cells and relate their structure to their electrical properties. Horseradish peroxidase (HRP), injected into these muscles, will be used to label their motoneurons for study with an electron microscope. A particular goal is to distinguish the structural basis for the different types or species of motoneurons in a single pool, which our physiological studies have revealed. Motoneurons of the cat's soleus (a homogeneous muscle) probably differ only in size, whereas those of plantaris, with three fiber types, probably differ in size and type. Comparison of cells in the two pools should help to distinguish different types. Mitochondria and HRP particles will be examined to determine whether their density or structure varies systematically with cell size or type. Anatomical clues that might lead to understanding of the differences in excitability of large and small cells will be especially sought. Motoneurons of equal size, but different firing rates, will be compared electrophysiologically to identify other distinguishing features, such as their response to fatigue and to a variety of inputs. The shape and size of individual EPSPs will be recorded intracellularly and averaged by the method of Mendell and Henneman. A spike-triggered averaging technique will be used to correlate fast and slow-firing units with the twitch contraction-times of their muscle fibers. Previous studies on inputs will be extended to define their role in the relation between motoneuron size and excitability. Although selective control over single motor units apparently does not occur normally, conditioning techniques will be used to learn whether it can be learned with prolonged training. Theoretical formulations of the size-principle by others will be analyzed and a comprehensive survey of the literature will be carried out to appraise the facts and theories regarding this general rule.